1. Field of the Invention
This invention relates to a tracking control device in a video disk player, a digital audio disk player, and so forth.
2. Background of the Invention
A conventional tracking device in a video disk player is shown in FIG. 1. In FIG. 1, a pair of detecting elements 1 such as photo-detectors (in the case of an optical disk player) are arranged on both sides of a track. The outputs of the detecting elements 1 are applied respectively through gain control circuits (AGC) 2 and 3 to a differential amplifier 4, which outputs a tracking error signal. The tracking error signal is applied through an addition circuit 5, an equalizer 6 and a drive circuit 7 to an actuator 8. The actuator 8 is adapted to drive an objective lens 8b through solenoid action thereby to cause a data detecting point to follow the track.
In the addition circuit 5, a warbling signal having a predetermined frequency which is outputted by an oscillating circuit 9 and phase-shifted by a phase-shifting circuit 10 as required is added to the tracking error signal, so that the data detecting point is warbled over a range, for instance, 0.05 to 0.1 .mu.m with respect to the track. A band-pass filter (BPF) 11 receives the output of the equalizer 6 and separates the warbling signal in the tracking servo loop from the output. The warbling signal thus separated, after being shaped by a comparisoncircuit 12, is applied to one input terminal of an exclusive-OR circuit 13, to the other terminal of which the warbling signal outputted by theoscillating circuit 9 is applied after being shaped by a comparison circuit 14. The two comparison circuits 12 and 14 compare their inputs against internal reference voltages and output binary signals dependent on whether the inputs are above or below the reference voltages. Possible values of the reference voltages are zero. The exclusive-OR circuit 13 subjects the warbling signal in the tracking servo loop to synchronous detection to detect and output a signal corresponding to the gain of the tracking servo loop. The detection output of the exclosure-OR circuit 13 is smoothed by a low-pass filter (LPF) 15, the output of which is applied to the gain control circuits 2 and 2 described above, so that the gain of the tracking servo loop (or the output levels of the detecting elements 1) is automatically controlled.
In the case where a special reproduction such as a still picture reproduction is carried out by causing the data detecting point to jump tracks, the data detecting point is moved from a track to the position between the tracks and from the position to the track. When the data detecting signal is warbled between the tracks, the polarity of the warbling signal is opposite to that of the warbling signal provided when it is warbled near the track. Therefore, in this case, a correct synchronous detection output cannot be obtained. However, in the conventional tracking control device, the gain control circuits 2 and 3 are operated as they are even when the data detecting point is positioned between the tracks. Therefore, the conventional tracking control device suffers from a difficulty that a correct gain control servo and accordingly a stable tracking servo cannot be obtained.